Probe to see if POSTing works

[mithro]

Don't keep trying to post if it will never work, such as when running under python -m SimpleHTTPServer 8000 rather then under `run-tests.py -b Chrome``

[mithro]

Temporary fix in https://github.com/dstockwell/web-animations-js/commit/dd5d006a9133b97584f423f09c9960695e612c4d